1. Field of the Invention
The present invention relates to an image-pickup apparatus configured to pick up a subject image.
2. Description of the Related Art
There have been many image-pickup apparatuses such as an electronic camera configured to record and/or reproduce a still image and/or a moving image picked up through a solid-state imaging element including a charge-coupled device (CCD), a complementary-metal-oxide-semiconductor (CMOS) sensor, etc.
According to the above-described image-pickup apparatuses, an output signal often fluctuates due to the voltage fluctuation of various types of powers for supply and the result thereof often appears on an output image. When an output signal occurs, the output signal being affected by gradual voltage fluctuation occurring at intervals longer than a time period required to read the data corresponding to a single line through an image-pickup element, the level fluctuates over plural lines. Therefore, vertical shading occurs on the output image. As the intervals at which the voltage fluctuation occurs become shorter, the vertical shading occurs more frequently so that horizontal stripes occur on the image.
The pattern in which the above-described vertical shading occurs due to the voltage fluctuation is not always constant. That is to say, the form of the vertical shading occurring on an image changes with each photographing. Therefore, it has been difficult to use correction data prepared in advance.
Therefore, as disclosed in Japanese Patent Laid-Open No. 2005-175930, vertical shading of which form changes with each imaging needs to be corrected by using an output value of an optically shielded horizontal optical-black (OB) area in the image-pickup element.
For example, there has been a method of calculating the average value of output signals of each line of the horizontal-OB area and subtracting the calculated average value from an output signal of the effective-pixel area of the line in a digital-signal-processing circuit such as an image engine.
Further, there has been a method of shifting an output signal of the effective-pixel area of the line for an output signal of the image-pickup unit by using a horizontal-OB-clamping unit of an analog-signal-processing circuit so that the average value of output signals of a horizontal-OB area of each line becomes the dark-reference value of the line.
Incidentally, OB pixels generating the horizontal-OB area include the following two types of OB pixels, as disclosed in Japanese Patent Laid-Open No. 2002-64196.
A first OB pixel includes a photoelectric-conversion element such as a photodiode and a layer provided to output a signal generated through the photoelectric-conversion element. The front face of the first OB pixel is optically shielded by using aluminum or the like.
A second OB pixel does not include the photoelectric-conversion element and/or includes the photoelectric-conversion element, but no layer provided to output the signal generated through the photoelectric-conversion element.
Being different from a signal output from the first OB pixel, a signal output from the second OB pixel does not include a dark-current component occurring in the photoelectric-conversion element. Therefore, it becomes possible to estimate the amount of dark current in the environment where photographing is performed by providing both the first and second OB pixels and comparing a signal output from the first OB pixel to that output from the second OB pixel. Further, no defect caused by the dark current is output from the second OB pixel and no noise component is output, the noise component occurring in each pixel. Therefore, a signal output from the second OB pixel can be used to correct another noise component which is not generated from any pixel.
On the other hand, a signal output from the first OB pixel includes the dark current, as is the case with an effective-pixel part receiving light from a subject. Therefore, a signal output from the first OB signal is required, so as to perform vertical-dark-shading correction to absorb the level fluctuation of a dark-current component of a signal output from the effective-pixel part.
However, when a strong noise occurs in any of the first OB pixels during the vertical-dark-shading correction performed by using the signal output from the first OB pixel, the correction precision is significantly affected by the noise. Therefore, under some photographing conditions, it may become difficult to perform the vertical-dark-shading correction with precision and the image quality may be deteriorated.
For example, if a long accumulation time is set for a little amount of incident light, noises included in signals output from the pixels are also increased. Therefore, variations in signals output from the first OB pixels may become significant. That is to say, the output-signal variations between the OB pixels may become significant. In that case, correction data generated from the signals output from the first OB pixels also varies from line to line. As a result, if correction is performed by using the correction data, an output image obtained after the correction may vary from line to line and horizontal stripes may occur on the image.
If photographing is performed under circumstances where much dark current occurs, that is to say, in a high-temperature environment and/or at the long-exposure time, the probability of occurrence of a defective pixel increases, where the defective pixel generates dark current of which amount is larger than that of dark current generated by pixels provided around the defective pixel. If the defective pixel exists, the average value of output signals of the first OB pixel of a line where the defective pixel exists is affected by the defective pixel, and the correction value of the above-described line becomes significantly different from those of other lines. Therefore, if horizontal-dark-shading correction is performed by using the correction data, improper correction is performed only for the line corresponding to the first OB pixel including the defective pixel. As a result, a horizontal line may occur on an image obtained after the correction.
In any case, if a sufficient number of first OB pixels are provided for each line and the number of the first OB pixels is increased, the first OB pixels being used to calculate the average value used to generate the correction data, variations in average values of the first OB pixels provided for each line can be decreased.
However, an unnecessary increase in the size of an area where the first OB pixels are arranged leads to an increase in the chip size so that the cost is increased. Therefore, the size of a horizontal-OB area where the first OB pixels are arranged is limited.
Therefore, if known vertical-dark-shading correction is performed, it is preferable that correction data is smoothed, so as to eliminate the effect of a noise component. However, even though the smoothed correction data does not affect correcting gradual vertical-dark shading, the use of the smoothed correction data makes it difficult to properly correct a vertical-shading component with a high frequency, such as the above-described horizontal stripes.
Thus, there is room for improvements in performing vertical-dark-shading correction with high precision independently of photographing conditions and/or a photographing environment.